Hydrogenolysis of C-Halogen Bonds

Breaking carbon-halogen bonds is very important because halogenated hydrocarbons are well-known environmental pollutants, and their fundamental 

Continuing this trend, vinylic bromine can be much more easily hydrogenolyzed than vinylic chlorine over a palladium catalyst.437 

A multiphase system consisting of a hydrocarbon solvent, a strong alkaline solution, and a quaternary onium salt, in the presence of a Pd/C catalyst with hydrogen that was bubbled at atmospheric pressure through the organic phase, allows the rapid displacement of chlorine from polyhalogenated benzenes. The onium salt, insoluble in both phases, is localized in the interfaces, coats the Pd/C catalyst, and constitutes the phase in which the reaction takes 

Hydrodehalogenations of chloro-, bromo-, and iodobenzene were carried out individually as well as in competitive reactions. When the reactions were carried out separately, the reduction of chlorobenzene closely paralleled that of bromobenzene, whereas the reduction of iodobenzene was slower. When they were allowed to react competitively, the reduction was highly selective, and the reaction was delayed, but iodobenzene reacted first followed by bromobenzene and then chlorobenzene. 

Carboxylic acid chlorides can be converted to aldehydes by hydrogenolysis on a poisoned Pd/BaS04 catalyst. This is the classic Rosenmund reduction. Over more active catalysts, further hydrogenation gives the corresponding alcohols. Bases are used to react with the HCl. For example, 2,5-dimethylpyridine was used as base in a Rosenmund reduction.457 

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Hydrogenolysis of the C-halogen bond is an important reaction both from preparative and from environmental points of view. [ HCo(CN)5]3 was studied in detail as a catalyst for reductive dehalogenations of organic halides, which proceed according to Eqs. (38) and (39). The results of the early experiments are summarized in [12]. 

Reductive cleavage of sigma bonds during catalytic hydrogenation is usually described as hydrogenolysis. Hydrogenolysis of C-O, C-N, and C-halogen bonds has been surveyed in different reviews [1-10]. 

Dehalogenation can occur by several reductive pathways. The simplest results in replacement of a C-bonded halogen atom with a hydrogen and is known as hydrogenolysis or reductive dehalogenation. The process is illustrated for trichloroethene, TCE,... 

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